We have "Schizo-Affective Disorder" in which we get to claim that the various "schizophrenias" and the various mood disorders are just one big unhappy diagnosis!
Makes things easy for the diagnostician - just one diagnosis, and you can prescribe lithium and anti-psychotics to everyone. And then you can pile on more drugs to treat the side-effects from the drug combinations you started with. And then of course, there are the side-effects of the side-effect treating drugs. Eventually you can work your way up to one or two dozen drugs at once.
Seriously - I have relatives that have suffered from this sort of diagnostic abuse.